Moineau style power sections are well known. They are useful in drilling motors for, e.g., subterranean drilling applications, in which they are used to covert a flow of drilling fluid into torque and rotary power. The general principle on which Moineau style power sections operate involves locating a helical rotor within a stator having a helical cavity. Helical cavity stators, when viewed in circular cross-section, show a series of peaks and valleys. The valleys are where the helical cavity is formed into the inside of the stator. The peaks are typically referred to as “lobes.”
The furthest outside diameter of the rotor is generally selected so as to allow the rotor to rotate within the stator while maintaining close proximity to the lobes on the stator. In most conventional Moineau style power sections, the rotor and the lobes on the stator are preferably an interference fit, with the rotor including one fewer lobes than the stator. Then, when fluid (such as drilling fluid) is passed through the helical spaces between rotor and stator, the flow of fluid causes the rotor to rotate.
Stators in Moineau style power sections typically show at least two components in circular cross-section. The outer portion includes a hollow cylindrical metal tube. The inner portion includes a helical cavity component. The helical cavities are formed in the inner surface of the helical cavity component. The helical cavity component also has a cylindrical outer surface that abuts the inner surface of the hollow metal tube.
Conventional stators in Moineau style power sections also advantageously include elastomer (e.g. rubber) surfaces on the inside of the helical cavities, and preferably on the lobes, to facilitate the interference fit with the rotor. The elastomer provides a resilient surface with which to contact the rotor as the rotor rotates. Many stators are known where the helical cavity component is made substantially entirely of elastomer.
It has been observed in operations using Moineau style power sections that the elastomer portions of the lobes are subject to considerable cyclic deflection. This deflection is caused not only by the interference fit with the rotor, but also by reactive torque from the rotor. The cyclic deflection and rebound of the elastomer causes a build up of heat in the elastomer. In conventional stators, especially those in which the helical cavity component is made substantially entirely from elastomer, the heat build up has been observed to concentrate near the center of the lobe. The heat build up weakens the elastomer, leading to a premature “chunking” breakdown of the elastomer. A cavity in the lobe also eventually develops as the deteriorated elastomer separates and falls away. This causes loss of lobe integrity, which causes loss of interference fit with the rotor, resulting in fluid leakage between rotor and stator as fluid is passed through the power sections. This fluid leakage in turn causes loss of drive torque, and if left unchecked will eventually lead to stalling of the rotor.
In other stators, such as described in exemplary embodiments disclosed in commonly-assigned, co-pending U.S. patent application Ser. No. 10/694,557, “COMPOSITE MATERIAL PROGRESSING CAVITY STATORS,” the elastomer may be a liner deployed on the helical cavity component, the helical cavity component comprising a fiber reinforced composite reinforcement material for the elastomer liner.
The deployment of a reinforcement material in the lobes addresses the problems of deterioration of an all-elastomer lobe due to heat build up. For example, lower resilience in the reinforcement material is likely to localize resilient displacement in the liner, where, in some embodiments, heat build up may dissipate more quickly. Care is required, however, in selection of reinforcement material and elastomer liner thickness. Contact stresses are caused on the reinforced lobes as the rotor rotates within the interference fit with the stator. Without sufficient resilience in the interference fit, the reinforcement may be too hard and/or the liner may be too thin, such that the contact stresses cause the elastomer liner to crack or split as the rotor contacts the stator lobe. Additionally, without care in choice of materials or elastomer liner thickness, the cyclic contact stresses can cause the lobes to crack or fail prematurely, particularly on the loaded side of the rotor/stator interface.